Welcome to Chapter 5 of the “Implementing a language with
LLVM” tutorial. Parts 1-4 described the implementation of
the simple Kaleidoscope language and included support for generating
LLVM IR, followed by optimizations and a JIT compiler. Unfortunately, as
presented, Kaleidoscope is mostly useless: it has no control flow other
than call and return. This means that you can’t have conditional
branches in the code, significantly limiting its power. In this episode
of “build that compiler”, we’ll extend Kaleidoscope to have an
if/then/else expression plus a simple ‘for’ loop.

Extending Kaleidoscope to support if/then/else is quite straightforward.
It basically requires adding lexer support for this “new” concept to the
lexer, parser, AST, and LLVM code emitter. This example is nice, because
it shows how easy it is to “grow” a language over time, incrementally
extending it as new ideas are discovered.

Before we get going on “how” we add this extension, lets talk about
“what” we want. The basic idea is that we want to be able to write this
sort of thing:

def fib(x)
if x < 3 then
1
else
fib(x-1)+fib(x-2);

In Kaleidoscope, every construct is an expression: there are no
statements. As such, the if/then/else expression needs to return a value
like any other. Since we’re using a mostly functional form, we’ll have
it evaluate its conditional, then return the ‘then’ or ‘else’ value
based on how the condition was resolved. This is very similar to the C
”?:” expression.

The semantics of the if/then/else expression is that it evaluates the
condition to a boolean equality value: 0.0 is considered to be false and
everything else is considered to be true. If the condition is true, the
first subexpression is evaluated and returned, if the condition is
false, the second subexpression is evaluated and returned. Since
Kaleidoscope allows side-effects, this behavior is important to nail
down.

Now that we know what we “want”, lets break this down into its
constituent pieces.

Now that we have it parsing and building the AST, the final piece is
adding LLVM code generation support. This is the most interesting part
of the if/then/else example, because this is where it starts to
introduce new concepts. All of the code above has been thoroughly
described in previous chapters.

To motivate the code we want to produce, lets take a look at a simple
example. Consider:

extern foo();
extern bar();
def baz(x) if x then foo() else bar();

If you disable optimizations, the code you’ll (soon) get from
Kaleidoscope looks like this:

To visualize the control flow graph, you can use a nifty feature of the
LLVM ‘opt‘ tool. If you put this LLVM
IR into “t.ll” and run “llvm-as<t.ll|opt-analyze-view-cfg”, a
window will pop up and you’ll
see this graph:

Example CFG

Another way to get this is to call
“Llvm_analysis.view_function_cfgf” or
“Llvm_analysis.view_function_cfg_onlyf” (where f is a
“Function”) either by inserting actual calls into the code and
recompiling or by calling these in the debugger. LLVM has many nice
features for visualizing various graphs.

Getting back to the generated code, it is fairly simple: the entry block
evaluates the conditional expression (“x” in our case here) and compares
the result to 0.0 with the “fcmpone” instruction (‘one’ is “Ordered
and Not Equal”). Based on the result of this expression, the code jumps
to either the “then” or “else” blocks, which contain the expressions for
the true/false cases.

Once the then/else blocks are finished executing, they both branch back
to the ‘ifcont’ block to execute the code that happens after the
if/then/else. In this case the only thing left to do is to return to the
caller of the function. The question then becomes: how does the code
know which expression to return?

The answer to this question involves an important SSA operation: the
Phi
operation.
If you’re not familiar with SSA, the wikipedia
article
is a good introduction and there are various other introductions to it
available on your favorite search engine. The short version is that
“execution” of the Phi operation requires “remembering” which block
control came from. The Phi operation takes on the value corresponding to
the input control block. In this case, if control comes in from the
“then” block, it gets the value of “calltmp”. If control comes from the
“else” block, it gets the value of “calltmp1”.

At this point, you are probably starting to think “Oh no! This means my
simple and elegant front-end will have to start generating SSA form in
order to use LLVM!”. Fortunately, this is not the case, and we strongly
advise not implementing an SSA construction algorithm in your
front-end unless there is an amazingly good reason to do so. In
practice, there are two sorts of values that float around in code
written for your average imperative programming language that might need
Phi nodes:

Code that involves user variables: x=1;x=x+1;

Values that are implicit in the structure of your AST, such as the
Phi node in this case.

In Chapter 7 of this tutorial (“mutable
variables”), we’ll talk about #1 in depth. For now, just believe me that
you don’t need SSA construction to handle this case. For #2, you have
the choice of using the techniques that we will describe for #1, or you
can insert Phi nodes directly, if convenient. In this case, it is really
really easy to generate the Phi node, so we choose to do it directly.

In order to generate code for this, we implement the Codegen method
for IfExprAST:

letreccodegen_expr=function...|Ast.If(cond,then_,else_)->letcond=codegen_exprcondin(* Convert condition to a bool by comparing equal to 0.0 *)letzero=const_floatdouble_type0.0inletcond_val=build_fcmpFcmp.Onecondzero"ifcond"builderin

This code is straightforward and similar to what we saw before. We emit
the expression for the condition, then compare that value to zero to get
a truth value as a 1-bit (bool) value.

(* Grab the first block so that we might later add the conditional branch * to it at the end of the function. *)letstart_bb=insertion_blockbuilderinletthe_function=block_parentstart_bbinletthen_bb=append_blockcontext"then"the_functioninposition_at_endthen_bbbuilder;

As opposed to the C++ tutorial, we have to build our
basic blocks bottom up since we can’t have dangling BasicBlocks. We
start off by saving a pointer to the first block (which might not be the
entry block), which we’ll need to build a conditional branch later. We
do this by asking the builder for the current BasicBlock. The fourth
line gets the current Function object that is being built. It gets this
by the start_bb for its “parent” (the function it is currently
embedded into).

Once it has that, it creates one block. It is automatically appended
into the function’s list of blocks.

(* Emit 'then' value. *)position_at_endthen_bbbuilder;letthen_val=codegen_exprthen_in(* Codegen of 'then' can change the current block, update then_bb for the * phi. We create a new name because one is used for the phi node, and the * other is used for the conditional branch. *)letnew_then_bb=insertion_blockbuilderin

We move the builder to start inserting into the “then” block. Strictly
speaking, this call moves the insertion point to be at the end of the
specified block. However, since the “then” block is empty, it also
starts out by inserting at the beginning of the block. :)

Once the insertion point is set, we recursively codegen the “then”
expression from the AST.

The final line here is quite subtle, but is very important. The basic
issue is that when we create the Phi node in the merge block, we need to
set up the block/value pairs that indicate how the Phi will work.
Importantly, the Phi node expects to have an entry for each predecessor
of the block in the CFG. Why then, are we getting the current block when
we just set it to ThenBB 5 lines above? The problem is that the “Then”
expression may actually itself change the block that the Builder is
emitting into if, for example, it contains a nested “if/then/else”
expression. Because calling Codegen recursively could arbitrarily change
the notion of the current block, we are required to get an up-to-date
value for code that will set up the Phi node.

(* Emit 'else' value. *)letelse_bb=append_blockcontext"else"the_functioninposition_at_endelse_bbbuilder;letelse_val=codegen_exprelse_in(* Codegen of 'else' can change the current block, update else_bb for the * phi. *)letnew_else_bb=insertion_blockbuilderin

Code generation for the ‘else’ block is basically identical to codegen
for the ‘then’ block.

The first two lines here are now familiar: the first adds the “merge”
block to the Function object. The second block changes the insertion
point so that newly created code will go into the “merge” block. Once
that is done, we need to create the PHI node and set up the block/value
pairs for the PHI.

(* Return to the start block to add the conditional branch. *)position_at_endstart_bbbuilder;ignore(build_cond_brcond_valthen_bbelse_bbbuilder);

Once the blocks are created, we can emit the conditional branch that
chooses between them. Note that creating new blocks does not implicitly
affect the IRBuilder, so it is still inserting into the block that the
condition went into. This is why we needed to save the “start” block.

(* Set a unconditional branch at the end of the 'then' block and the * 'else' block to the 'merge' block. *)position_at_endnew_then_bbbuilder;ignore(build_brmerge_bbbuilder);position_at_endnew_else_bbbuilder;ignore(build_brmerge_bbbuilder);(* Finally, set the builder to the end of the merge block. *)position_at_endmerge_bbbuilder;phi

To finish off the blocks, we create an unconditional branch to the merge
block. One interesting (and very important) aspect of the LLVM IR is
that it requires all basic blocks to be
“terminated” with a control flow
instruction such as return or branch.
This means that all control flow, including fall throughs must be made
explicit in the LLVM IR. If you violate this rule, the verifier will
emit an error.

Finally, the CodeGen function returns the phi node as the value computed
by the if/then/else expression. In our example above, this returned
value will feed into the code for the top-level function, which will
create the return instruction.

Overall, we now have the ability to execute conditional code in
Kaleidoscope. With this extension, Kaleidoscope is a fairly complete
language that can calculate a wide variety of numeric functions. Next up
we’ll add another useful expression that is familiar from non-functional
languages...

This expression defines a new variable (“i” in this case) which iterates
from a starting value, while the condition (“i < n” in this case) is
true, incrementing by an optional step value (“1.0” in this case). If
the step value is omitted, it defaults to 1.0. While the loop is true,
it executes its body expression. Because we don’t have anything better
to return, we’ll just define the loop as always returning 0.0. In the
future when we have mutable variables, it will get more useful.

As before, lets talk about the changes that we need to Kaleidoscope to
support this.

...inToken.token...(* control *)|If|Then|Else|For|In...inLexer.lex_ident...matchBuffer.contentsbufferwith|"def"->[<'Token.Def;stream>]|"extern"->[<'Token.Extern;stream>]|"if"->[<'Token.If;stream>]|"then"->[<'Token.Then;stream>]|"else"->[<'Token.Else;stream>]|"for"->[<'Token.For;stream>]|"in"->[<'Token.In;stream>]|id->[<'Token.Identid;stream>]

The parser code is also fairly standard. The only interesting thing here
is handling of the optional step value. The parser code handles it by
checking to see if the second comma is present. If not, it sets the step
value to null in the AST node:

Now we get to the good part: the LLVM IR we want to generate for this
thing. With the simple example above, we get this LLVM IR (note that
this dump is generated with optimizations disabled for clarity):

With this out of the way, the next step is to set up the LLVM basic
block for the start of the loop body. In the case above, the whole loop
body is one block, but remember that the body code itself could consist
of multiple blocks (e.g. if it contains an if/then/else or a for/in
expression).

(* Make the new basic block for the loop header, inserting after current * block. *)letpreheader_bb=insertion_blockbuilderinletthe_function=block_parentpreheader_bbinletloop_bb=append_blockcontext"loop"the_functionin(* Insert an explicit fall through from the current block to the * loop_bb. *)ignore(build_brloop_bbbuilder);

This code is similar to what we saw for if/then/else. Because we will
need it to create the Phi node, we remember the block that falls through
into the loop. Once we have that, we create the actual block that starts
the loop and create an unconditional branch for the fall-through between
the two blocks.

(* Start insertion in loop_bb. *)position_at_endloop_bbbuilder;(* Start the PHI node with an entry for start. *)letvariable=build_phi[(start_val,preheader_bb)]var_namebuilderin

Now that the “preheader” for the loop is set up, we switch to emitting
code for the loop body. To begin with, we move the insertion point and
create the PHI node for the loop induction variable. Since we already
know the incoming value for the starting value, we add it to the Phi
node. Note that the Phi will eventually get a second value for the
backedge, but we can’t set it up yet (because it doesn’t exist!).

(* Within the loop, the variable is defined equal to the PHI node. If it * shadows an existing variable, we have to restore it, so save it * now. *)letold_val=trySome(Hashtbl.findnamed_valuesvar_name)withNot_found->NoneinHashtbl.addnamed_valuesvar_namevariable;(* Emit the body of the loop. This, like any other expr, can change the * current BB. Note that we ignore the value computed by the body, but * don't allow an error *)ignore(codegen_exprbody);

Now the code starts to get more interesting. Our ‘for’ loop introduces a
new variable to the symbol table. This means that our symbol table can
now contain either function arguments or loop variables. To handle this,
before we codegen the body of the loop, we add the loop variable as the
current value for its name. Note that it is possible that there is a
variable of the same name in the outer scope. It would be easy to make
this an error (emit an error and return null if there is already an
entry for VarName) but we choose to allow shadowing of variables. In
order to handle this correctly, we remember the Value that we are
potentially shadowing in old_val (which will be None if there is no
shadowed variable).

Once the loop variable is set into the symbol table, the code
recursively codegen’s the body. This allows the body to use the loop
variable: any references to it will naturally find it in the symbol
table.

(* Emit the step value. *)letstep_val=matchstepwith|Somestep->codegen_exprstep(* If not specified, use 1.0. *)|None->const_floatdouble_type1.0inletnext_var=build_addvariablestep_val"nextvar"builderin

Now that the body is emitted, we compute the next value of the iteration
variable by adding the step value, or 1.0 if it isn’t present.
‘next_var‘ will be the value of the loop variable on the next
iteration of the loop.

(* Compute the end condition. *)letend_cond=codegen_exprend_in(* Convert condition to a bool by comparing equal to 0.0. *)letzero=const_floatdouble_type0.0inletend_cond=build_fcmpFcmp.Oneend_condzero"loopcond"builderin

Finally, we evaluate the exit value of the loop, to determine whether
the loop should exit. This mirrors the condition evaluation for the
if/then/else statement.

(* Create the "after loop" block and insert it. *)letloop_end_bb=insertion_blockbuilderinletafter_bb=append_blockcontext"afterloop"the_functionin(* Insert the conditional branch into the end of loop_end_bb. *)ignore(build_cond_brend_condloop_bbafter_bbbuilder);(* Any new code will be inserted in after_bb. *)position_at_endafter_bbbuilder;

With the code for the body of the loop complete, we just need to finish
up the control flow for it. This code remembers the end block (for the
phi node), then creates the block for the loop exit (“afterloop”). Based
on the value of the exit condition, it creates a conditional branch that
chooses between executing the loop again and exiting the loop. Any
future code is emitted in the “afterloop” block, so it sets the
insertion position to it.

(* Add a new entry to the PHI node for the backedge. *)add_incoming(next_var,loop_end_bb)variable;(* Restore the unshadowed variable. *)beginmatchold_valwith|Someold_val->Hashtbl.addnamed_valuesvar_nameold_val|None->()end;(* for expr always returns 0.0. *)const_nulldouble_type

The final code handles various cleanups: now that we have the
“next_var” value, we can add the incoming value to the loop PHI
node. After that, we remove the loop variable from the symbol table, so
that it isn’t in scope after the for loop. Finally, code generation of
the for loop always returns 0.0, so that is what we return from
Codegen.codegen_expr.

With this, we conclude the “adding control flow to Kaleidoscope” chapter
of the tutorial. In this chapter we added two control flow constructs,
and used them to motivate a couple of aspects of the LLVM IR that are
important for front-end implementors to know. In the next chapter of our
saga, we will get a bit crazier and add user-defined
operators to our poor innocent language.

(*===----------------------------------------------------------------------=== * Lexer Tokens *===----------------------------------------------------------------------===*)(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of * these others for known things. *)typetoken=(* commands *)|Def|Extern(* primary *)|Identofstring|Numberoffloat(* unknown *)|Kwdofchar(* control *)|If|Then|Else|For|In

lexer.ml:

(*===----------------------------------------------------------------------=== * Lexer *===----------------------------------------------------------------------===*)letreclex=parser(* Skip any whitespace. *)|[<'(' '|'\n'|'\r'|'\t');stream>]->lexstream(* identifier: [a-zA-Z][a-zA-Z0-9] *)|[<'('A'..'Z'|'a'..'z'asc);stream>]->letbuffer=Buffer.create1inBuffer.add_charbufferc;lex_identbufferstream(* number: [0-9.]+ *)|[<'('0'..'9'asc);stream>]->letbuffer=Buffer.create1inBuffer.add_charbufferc;lex_numberbufferstream(* Comment until end of line. *)|[<'('#');stream>]->lex_commentstream(* Otherwise, just return the character as its ascii value. *)|[<'c;stream>]->[<'Token.Kwdc;lexstream>](* end of stream. *)|[<>]->[<>]andlex_numberbuffer=parser|[<'('0'..'9'|'.'asc);stream>]->Buffer.add_charbufferc;lex_numberbufferstream|[<stream=lex>]->[<'Token.Number(float_of_string(Buffer.contentsbuffer));stream>]andlex_identbuffer=parser|[<'('A'..'Z'|'a'..'z'|'0'..'9'asc);stream>]->Buffer.add_charbufferc;lex_identbufferstream|[<stream=lex>]->matchBuffer.contentsbufferwith|"def"->[<'Token.Def;stream>]|"extern"->[<'Token.Extern;stream>]|"if"->[<'Token.If;stream>]|"then"->[<'Token.Then;stream>]|"else"->[<'Token.Else;stream>]|"for"->[<'Token.For;stream>]|"in"->[<'Token.In;stream>]|id->[<'Token.Identid;stream>]andlex_comment=parser|[<'('\n');stream=lex>]->stream|[<'c;e=lex_comment>]->e|[<>]->[<>]

(*===----------------------------------------------------------------------=== * Code Generation *===----------------------------------------------------------------------===*)openLlvmexceptionErrorofstringletcontext=global_context()letthe_module=create_modulecontext"my cool jit"letbuilder=buildercontextletnamed_values:(string,llvalue)Hashtbl.t=Hashtbl.create10letdouble_type=double_typecontextletreccodegen_expr=function|Ast.Numbern->const_floatdouble_typen|Ast.Variablename->(tryHashtbl.findnamed_valuesnamewith|Not_found->raise(Error"unknown variable name"))|Ast.Binary(op,lhs,rhs)->letlhs_val=codegen_exprlhsinletrhs_val=codegen_exprrhsinbeginmatchopwith|'+'->build_addlhs_valrhs_val"addtmp"builder|'-'->build_sublhs_valrhs_val"subtmp"builder|'*'->build_mullhs_valrhs_val"multmp"builder|'<'->(* Convert bool 0/1 to double 0.0 or 1.0 *)leti=build_fcmpFcmp.Ultlhs_valrhs_val"cmptmp"builderinbuild_uitofpidouble_type"booltmp"builder|_->raise(Error"invalid binary operator")end|Ast.Call(callee,args)->(* Look up the name in the module table. *)letcallee=matchlookup_functioncalleethe_modulewith|Somecallee->callee|None->raise(Error"unknown function referenced")inletparams=paramscalleein(* If argument mismatch error. *)ifArray.lengthparams==Array.lengthargsthen()elseraise(Error"incorrect # arguments passed");letargs=Array.mapcodegen_exprargsinbuild_callcalleeargs"calltmp"builder|Ast.If(cond,then_,else_)->letcond=codegen_exprcondin(* Convert condition to a bool by comparing equal to 0.0 *)letzero=const_floatdouble_type0.0inletcond_val=build_fcmpFcmp.Onecondzero"ifcond"builderin(* Grab the first block so that we might later add the conditional branch * to it at the end of the function. *)letstart_bb=insertion_blockbuilderinletthe_function=block_parentstart_bbinletthen_bb=append_blockcontext"then"the_functionin(* Emit 'then' value. *)position_at_endthen_bbbuilder;letthen_val=codegen_exprthen_in(* Codegen of 'then' can change the current block, update then_bb for the * phi. We create a new name because one is used for the phi node, and the * other is used for the conditional branch. *)letnew_then_bb=insertion_blockbuilderin(* Emit 'else' value. *)letelse_bb=append_blockcontext"else"the_functioninposition_at_endelse_bbbuilder;letelse_val=codegen_exprelse_in(* Codegen of 'else' can change the current block, update else_bb for the * phi. *)letnew_else_bb=insertion_blockbuilderin(* Emit merge block. *)letmerge_bb=append_blockcontext"ifcont"the_functioninposition_at_endmerge_bbbuilder;letincoming=[(then_val,new_then_bb);(else_val,new_else_bb)]inletphi=build_phiincoming"iftmp"builderin(* Return to the start block to add the conditional branch. *)position_at_endstart_bbbuilder;ignore(build_cond_brcond_valthen_bbelse_bbbuilder);(* Set a unconditional branch at the end of the 'then' block and the * 'else' block to the 'merge' block. *)position_at_endnew_then_bbbuilder;ignore(build_brmerge_bbbuilder);position_at_endnew_else_bbbuilder;ignore(build_brmerge_bbbuilder);(* Finally, set the builder to the end of the merge block. *)position_at_endmerge_bbbuilder;phi|Ast.For(var_name,start,end_,step,body)->(* Emit the start code first, without 'variable' in scope. *)letstart_val=codegen_exprstartin(* Make the new basic block for the loop header, inserting after current * block. *)letpreheader_bb=insertion_blockbuilderinletthe_function=block_parentpreheader_bbinletloop_bb=append_blockcontext"loop"the_functionin(* Insert an explicit fall through from the current block to the * loop_bb. *)ignore(build_brloop_bbbuilder);(* Start insertion in loop_bb. *)position_at_endloop_bbbuilder;(* Start the PHI node with an entry for start. *)letvariable=build_phi[(start_val,preheader_bb)]var_namebuilderin(* Within the loop, the variable is defined equal to the PHI node. If it * shadows an existing variable, we have to restore it, so save it * now. *)letold_val=trySome(Hashtbl.findnamed_valuesvar_name)withNot_found->NoneinHashtbl.addnamed_valuesvar_namevariable;(* Emit the body of the loop. This, like any other expr, can change the * current BB. Note that we ignore the value computed by the body, but * don't allow an error *)ignore(codegen_exprbody);(* Emit the step value. *)letstep_val=matchstepwith|Somestep->codegen_exprstep(* If not specified, use 1.0. *)|None->const_floatdouble_type1.0inletnext_var=build_addvariablestep_val"nextvar"builderin(* Compute the end condition. *)letend_cond=codegen_exprend_in(* Convert condition to a bool by comparing equal to 0.0. *)letzero=const_floatdouble_type0.0inletend_cond=build_fcmpFcmp.Oneend_condzero"loopcond"builderin(* Create the "after loop" block and insert it. *)letloop_end_bb=insertion_blockbuilderinletafter_bb=append_blockcontext"afterloop"the_functionin(* Insert the conditional branch into the end of loop_end_bb. *)ignore(build_cond_brend_condloop_bbafter_bbbuilder);(* Any new code will be inserted in after_bb. *)position_at_endafter_bbbuilder;(* Add a new entry to the PHI node for the backedge. *)add_incoming(next_var,loop_end_bb)variable;(* Restore the unshadowed variable. *)beginmatchold_valwith|Someold_val->Hashtbl.addnamed_valuesvar_nameold_val|None->()end;(* for expr always returns 0.0. *)const_nulldouble_typeletcodegen_proto=function|Ast.Prototype(name,args)->(* Make the function type: double(double,double) etc. *)letdoubles=Array.make(Array.lengthargs)double_typeinletft=function_typedouble_typedoublesinletf=matchlookup_functionnamethe_modulewith|None->declare_functionnameftthe_module(* If 'f' conflicted, there was already something named 'name'. If it * has a body, don't allow redefinition or reextern. *)|Somef->(* If 'f' already has a body, reject this. *)ifblock_beginf<>At_endfthenraise(Error"redefinition of function");(* If 'f' took a different number of arguments, reject. *)ifelement_type(type_off)<>ftthenraise(Error"redefinition of function with different # args");fin(* Set names for all arguments. *)Array.iteri(funia->letn=args.(i)inset_value_namena;Hashtbl.addnamed_valuesna;)(paramsf);fletcodegen_functhe_fpm=function|Ast.Function(proto,body)->Hashtbl.clearnamed_values;letthe_function=codegen_protoprotoin(* Create a new basic block to start insertion into. *)letbb=append_blockcontext"entry"the_functioninposition_at_endbbbuilder;tryletret_val=codegen_exprbodyin(* Finish off the function. *)let_=build_retret_valbuilderin(* Validate the generated code, checking for consistency. *)Llvm_analysis.assert_valid_functionthe_function;(* Optimize the function. *)let_=PassManager.run_functionthe_functionthe_fpminthe_functionwithe->delete_functionthe_function;raisee